1. Field of the Invention
This invention relates generally to comparators used to determine when a signal exceeds a predetermine setpoint. This invention relates specifically to comparators used to determine when a signal of unknown polarity exceeds a predetermined change in value. In particular, the present invention relates to comparators used to determine when transducers, such as pyroelectric sensors or ultrasonic/acoustic transducers, used for example as motion sensors, have detected the occurrence of an event they have been designed to detect. Such transducers may be used as occupancy sensors to control lighting, heating, cooling and/or other systems in response to the occupancy of a room.
2. Description of the Prior Art
Conventional transducers used as motion and/or occupancy sensors may operate passively, by detecting changes in ambient conditions such as changes in temperature, occasioned by the entry of a warm bodied being into a room. Alternatively, conventional transducers may operate actively by flooding the environment to be monitored with transmitted and reflected energy and then detecting changes in the resultant steady state energy reception pattern observed by the transducer from the monitored environment in order to detect the event to be monitored.
Conventional occupancy sensors tend to be sensitive to changes in ambient conditions not intended to be detected, such as changes in temperature and even air densities, that interfere with their ability to accurately detect events to be monitored. When used as occupancy detectors, for example, the transducers must be sufficiently sensitive to detect minor motions, such as the movement of a human arm through a specified arc, without falsely being triggered by other events such as changes in the ambient conditions occasioned by thermal or other drift.
The polarity of the change of the output signal of a transducer used in an occupancy sensor will typically not be known before it occurs because the motions or other events to be detected may occur in any direction. The event to be detected cannot therefore be conveniently determined by comparison against a single set point because the direction of change of the signal, that is, the polarity of the transducer signal, cannot be predicted before the event. The transducer output signal must therefore be compared against set points for each polarity, typically both an upper and a lower setpoint value. If the transducer output is above the upper setpoint or below the lower setpoint, the event to be monitored is indicated to be detected. Conventionally, the outputs of transducers used in occupancy sensors are applied to a window comparator to determine if the transducer signal exceeds the upper or lower threshold or setpoint.
Systems using conventional circuits, such as window comparators, are especially susceptible to errors including false detections, from thermal drift and other long term ambient changes. Such ambient changes may mask the signal to be detected and may be caused by temperature changes during the day, changes in the operation of heating and air conditioning equipment as well as even changes in humidity.
What are needed are convenient techniques suitable for detecting events of unknown signal polarities from the outputs of transducers used, for example, in occupancy sensors.